Fix attiny84 and possible other chips without hardware UART

optiboot/bootloaders/optiboot/pin_defs.h

@@ -42,6 +42,7 @@
  * Note that mega8/m32 still needs special handling, because ubrr is handled
  * differently.
  */
+#ifndef SOFT_UART
 #if UART == 0
 #if defined(UDR0)
 # define UART_SRA UCSR0A
@@ -93,6 +94,7 @@
 # define UART_SRL UBRR3L
 # define UART_UDR UDR3
 #endif
+#endif
 
 #if    defined(__AVR_ATmega8__)		\
     || defined (__AVR_ATmega32__)	\